Currently, halogen-containing flame retardants (especially brominated flame retardants) are widely applied in macromolecular flame retardant materials, and have a better flame retardant effect. With the improvement of people's awareness of environmental protection and the promulgation of a series of EU directives, the development of halogen-free flame retardant printed wiring boards has become a key point in the industry. The manufacturers of copper-clad laminates all launch their own halogen-free flame retardant copper-clad laminates.
Phosphorus-containing resins are generally used in the current industry to realize the flame retardant effect. However, too much phosphorus introduction will make the substrate have a high water-absorption, a worse chemical resistance and the like.
In recent years, benzoxazine as matrix resin for the development of halogen-free substrate has drawn more attention. Benzoxazine is a benzo six-membered heterocyclic system consisting of oxygen atoms and nitrogen atoms, and has a feature of ring-opening polymerization. During the polymerization, no micromolecule is released; after the polymerization, a reticulate structure like phenolic resin is formed. The product has a small curing shrinkage, a low porosity, excellent mechanical, electrical and flame retardant properties.
On the other hand, electronic products are developed in the direction of lightness, thinness, shortness, high densification, securitization and high functionalization along with rapid development of electronic industry, which imposes higher performance requirements on the printed circuit boards as a carrier. With the high speed and multi-functionalization of information processing of electronic products, and the continuous improvement of application frequency, the requirements on the dielectric constant and dielectric loss will become lower, besides maintaining higher requirements on the heat resistance of the laminate materials.
The current conventional FR-4 cannot satisfy the use requirements on high frequency and rapid development of electronic products. Meanwhile, the substrate materials, together with electronic components, will become one important route for the designers of PCB and terminal manufacturers to improve the product performances, rather than play a role of traditional mechanical support any more. High Dk will slow the signal transmission rate, and high Df will partially transform the signal to thermal energy and loss in the substrate materials. Thus high frequency transmission having a low dielectric constant and a low dielectric loss, especially the development of halogen-free high frequency plates, has become a key point of the industry of copper-clad laminates.
In order to solve the aforesaid problems, CN101684191B discloses obtaining a cured product having a lower dielectric constant and dielectric loss by co-curing epoxy resin with benzoxazine, styrene-maleic anhydride (SMA) and phosphorus-containing curing agent. Although too much SMA can reduce the dielectric constant of the material, there will be other unavoidable problems, especially notable effect on cohesiveness. This is because non-polar styrene structural units in SMA molecular structure decrease the polarity of the modified matrix resin and weaken the interaction force between resins and copper foils. Meanwhile, plenty of benzene ring structures in SMA increase the fragility of resin cross-linked network, which will have adverse effect on the cohesiveness under dynamic conditions, so as to reduce the binding strength between the substrates and between the substrates and copper foils. Along with the development of electronic industry, higher requirements on heat resistance of substrates are put forward.
CN100523081C discloses co-curing phosphorus-containing and halogen-free, phosphorus-free epoxy composition with benzoxazine, styrene-maleic anhydride and other curing agents to obtain a cured product having a lower dielectric constant and dielectric loss. Although phosphorus-containing epoxy as the matrix resin can achieve excellent flame retardant, the introduction of too much phosphorus will necessarily have a great effect on the water absorption of the substrate, and have adverse effect on many performances of the substrate.
CN103131131A discloses co-curing epoxy resin with benzoxazine, styrene-maleic anhydride and amine curing agent to obtain a cured product having a lower dielectric constant and dielectric loss. Although the introduction of amine curing agent can increase the cohesiveness of the substrate, the heat resistance of the substrate will necessarily be reduced, which cannot meet the requirements on high multi-layer applications.
CN 103881302A discloses a resin composition and copper foil substrate and printed circuit board using the same. The resin composition comprises epoxy resins, benzoxazine resins, styrene-maleic anhydride copolymers and polyesters. Although polyesters have better dielectric properties, they have a high price. Moreover, too much benzoxazine is used in the resin composition, which will increase the fragility of the substrate.
Moreover, the prior art above of co-curing epoxy resins with benzoxazine and styrene-maleic anhydride does not consider the effect of the usage amounts of benzoxazine and styrene-maleic anhydride on the dielectric loss value (Df) stability of the prepreg under different curing temperature conditions. Stable Df value has an important meaning to the stable transmission of signals in the terminal application of substrates.